The present invention relates to an evaporator in a refrigeration system. More particularly, the present invention relates to a falling film evaporator for a vapor compression refrigeration chiller.
At its simplest, a vapor compression refrigeration chiller includes a compressor, a condenser, an expansion device and an evaporator. Refrigerant gas is compressed in and is delivered from the compressor to the condenser, at a relatively high pressure, where the it is cooled and condensed to the liquid state. The condensed refrigerant passes from the condenser to and through the expansion device. Passage of the refrigerant through the expansion device causes a pressure drop therein and the further cooling thereof. As a result, the refrigerant delivered from the expansion device to the evaporator is generally a relatively cool, saturated two-phase mixture.
The two-phase refrigerant mixture delivered to the evaporator is brought into contact with a tube bundle disposed therein and through which a relatively warmer heat transfer medium, such as water, flows. That medium will have been warmed by heat exchange contact with the heat load which it is the purpose of the refrigeration chiller to cool.
Heat exchange contact between the relatively cool refrigerant and the relatively warm heat transfer medium flowing through the tube bundle causes the refrigerant to vaporize and the heat transfer medium to be cooled. The now cooled medium is returned to the heat load to further cool it while the heated and now vaporized refrigerant is directed out of the evaporator and is drawn into the chiller's compressor for recompression and delivery to the condenser in a continuous process.
More recently, environmental, efficiency and other similar issues and concerns have resulted in a need to re-think evaporator design in vapor compression refrigeration chillers in view of making such evaporators more efficient, from a heat exchange efficiency standpoint, and in view of reducing the size of the refrigerant charge needed in such chillers. In that regard, environmental circumstances relating to ozone depletion and global warming have taken on significant importance in the past several years. Those issues and the ramifications thereof have driven both a need to reduce the amount and change the nature of refrigerants used in refrigeration chillers.
So-called falling film evaporators have, for some time, been identified as promising candidates for use in refrigeration chillers to address efficiency, environmental and other issues and concerns in the nature of those referred to above. While the use and application of evaporators of a falling film design in vapor compression refrigeration chillers is theoretically beneficial, their design, manufacture and incorporation into such chiller systems has proven challenging.
In traditional shell-and-tube flooded evaporators, the shell of the evaporator is largely filled with liquid refrigerant and a majority of the tubes in the tube bundle are immersed therein. Two-phase refrigerant is directed upward to the evaporator's tube bundle from a distributor located at the bottom of the shell. Refrigerant vapor generated in such evaporators entrains liquid refrigerant droplets and carries them upward to the uppermost, unimmersed rows of tubes within the tube bundle for heat exchange therewith. Good axial distribution of the two-phase refrigerant mixture within the shell is important to ensure that the tube bundle is and remains fully wetted. As will be appreciated, flooded evaporators, by their nature, require that the chiller system employ a relatively large refrigerant charge.
One recent attempt to address issues relating to the amount of refrigerant used in a refrigeration system is identified in U.S. Pat. No. 5,839,294 which suggests the employment of what it refers to as a "hybrid" falling film evaporator. Despite the reference to this evaporator as a form of falling film evaporator, the '294 patent states that in its preferred embodiment, about one-half of the tubes in its tube bundle are immersed in liquid refrigerant and that in some cases, up to three quarters of the tube bundle would be. Further, that patent teaches and relies upon the use of pressure and spray heads or nozzles to distribute refrigerant onto the portion of the tubes in the tube bundle that are not immersed in liquid refrigerant. The use of pressure to spray liquid refrigerant onto a tube bundle penalizes the efficiency of the heat exchange process due to the fact that a portion of the liquid refrigerant in the spray will be carried out of the evaporator in the stream of refrigerant gas that flows to the compressor therefrom without having come into heat exchange contact with a heat exchanger tube internal of the evaporator. Further, when pressurized or spray systems are used, a larger amount of liquid refrigerant will fall into the evaporator's liquid pool without contacting a heat exchanger tube than will be the case in true or non-hybrid falling film evaporators.
Non-hybrid falling film evaporators go significantly further to reduce the amount of refrigerant needed for efficient evaporator and chiller system operation by virtue of the fact that relatively very little liquid refrigerant is carried out of the evaporator entrained in the refrigerant gas that flows out of the evaporator to the compressor and significantly less refrigerant makes its way to the bottom of the evaporator shell without having come into heat exchange contact with a tube in the tube bundle. Still further, only a relatively small portion of the tubes in the tube bundle are immersed in the relatively shallow pool of liquid refrigerant that does collect at the bottom of the evaporator shell.
In true falling film evaporators, liquid refrigerant is deposited, preferably in a low-energy, gentle fashion, onto the evaporator's tube bundle from above and gravity is relied upon to cause liquid refrigerant to fall generally vertically downward through the bundle in droplet and film form. Because of these characteristics, falling film evaporators require a reduced amount of refrigerant to function and will typically provide superior thermal performance to that of flooded and/or hybrid evaporators due to the improved heat transfer coefficient that results from the creation of the thin film of liquid refrigerant that flows over and around the majority of the individual tubes in the tube bundle. Further, evaporator efficiency and performance is improved as a result of the elimination of the adverse hydrostatic head effects caused by the relatively more large and deep pool of liquid refrigerant which is found in evaporators of the flooded type.
With respect to falling film evaporators, in operation, the vaporization of refrigerant liquid within the tube bundle of such evaporators generates vapor which tends to travel generally upward but along the path of least resistance in order to exit the but bundle. Because the refrigerant delivered onto a tube bundle in a falling film evaporator is from above and because such delivery requires the use of distributor apparatus to provide for the uniform distribution and deposit of refrigerant onto the tube bundle, generally along its entire length and width, refrigerant vapor generated in the tube bundle, which will naturally tend to rise, must be conducted both vertically and horizontally out of the tube bundle and around the refrigerant distributor so as to conduct it to a location from where it can be drawn from the evaporator into the system's compressor.
The specific vapor flow path in a tube bundle is affected by bundle geometry, tube patterns and by flow conditions therein, including vapor buoyancy effects. Managing vapor flow within the tube bundle of a falling film evaporator is therefore of significant importance to the efficiency of the heat exchange process that occurs therein as is ensuring that the flow of refrigerant, when it is initially received from the distributor at the top of the tube bundle, is "evened out" for downward flow therethrough.
If the downward flow of liquid refrigerant as it initially occurs in the upper portion of the tube bundle is not "evened out" thereacross, the efficiency of the heat transfer process within the evaporator and of the vapor compression refrigeration chiller as a whole will be degraded by oversupply of liquid refrigerant to one portion of the bundle and undersupply to another. Further, if local vapor velocity within the tube bundle becomes too high, particularly in a direction which is laterally across the tube bundle, breakdown of the film of liquid refrigerant that develops around individual tubes and the existence of which is critical to the heat transfer process can occur. Such breakdown can lead to the existence of localized dry regions in the tube bundle. The existence of such localized dry regions, or "dry out" as it is referred to, like maldistribution of the liquid refrigerant as it is initially received at the top of the tube bundle, degrades the overall heat transfer performance of a falling film evaporator.
Exemplary of the use of a true, non-hybrid falling film evaporators in vapor compression refrigeration chillers is the relatively new, so-called RTHC chiller manufactured by the assignee of the present invention. Reference may be had to U.S. Pat. Nos. 5,645,124; 5,638,691 and 5,588,596, likewise assigned to the assignee of the present invention and all of which derive from a single U.S. patent application, for their description of early efforts as they relate to the design of falling film evaporators for use in vapor compression refrigeration chillers and of refrigerant distribution systems therefor. Reference may also be had to U.S. Pat. Nos. 5,561,987 and 5,761,914, likewise assigned to the assignee of the present invention, which similarly relate to chiller systems that makes use of a falling film evaporator.
In the RTHC chiller, which is currently state of the art in the industry, the tube bundle can be categorized as being generally homogenous in terms of its tube patterns and tube bundle geometry. Proactive control of the flow of refrigerant vapor generated within the tube bundle of the RTHC chiller is not critical for the reason that a dedicated liquidvapor separator component is employed in that chiller, upstream of the evaporator's refrigerant distributor. As a result of the use of such a dedicated liquid-vapor separator component, the refrigerant delivered into the distributor within the evaporator of the RTHC chiller is in the liquid phase only. As a result of the need to distribute only liquid phase refrigerant onto the tube bundle within the RTHC evaporator, the distributor therein is of a design which does not generally inhibit the upward flow of refrigerant vapor upward and out of the evaporator. The requirement for and use of a dedicated liquid-vapor separator component does come, however, at significant expense in terms of chiller material and fabrication costs.
More recently, a refrigerant distributor of a new and highly efficient design has been developed by which the generally controlled and predictable distribution of a two-phase, vapor-liquid refrigerant mixture within a falling film evaporator in a vapor compression refrigeration system is successfully accomplished. That two-phase refrigerant distributor is the subject of co-assigned and co-pending U.S. patent application Ser. No. 09/267,413 filed on Mar. 12, 1999. The efficiency and effectiveness of this two-phase distributor has eliminated the need for a separate liquid-vapor separator component in chillers that employ falling film evaporators. While elimination of the dedicated and expensive liquid-vapor separator component is very clearly beneficial, it does come at the cost of adding some complexity and design difficulties to the overall evaporator design.
In that regard, in order for a distributor to accomplish efficient and even distribution of two-phase refrigerant to the tube bundle in a falling film evaporator, it will typically be of a generally solid and impervious design that will overlie the majority of the length and width of the evaporator's tube bundle. Distributors of such a design do not, therefore, generally facilitate the unobstructed vertical flow of refrigerant vapor to and out of the upper region of the evaporator.
Because the two-phase refrigerant distributor is a generally impervious component that overlies the majority of the length and width of the tube bundle, refrigerant vapor generated within the tube bundle must be caused to flow horizontally, in a cross-flow direction with respect to the downward flow of liquid refrigerant through the tube bundle, in order to conduct such vapor to the sides of the tube bundle from where it can be drawn upward and out of the evaporator shell unobstructed by the distributor. Such flow must be managed to minimize both the disruption of the distribution of refrigerant out of the distributor onto the top of the tube bundle and the downward flow of liquid refrigerant through the tube bundle.
The need therefore exists for a falling film evaporator for use in a vapor compression refrigeration system in which the need for a dedicated liquid-vapor separator component is obviated by the use of a two-phase refrigerant distributor yet which provides for the pro-active control of the flow of refrigerant vapor within and out of its tube bundle and shell in a manner which minimizes the disruption of the distribution of refrigerant onto the tube bundle from above and the downward flow of liquid refrigerant therethrough.